Thursday, May 21, 2009

Name Matching Alternatives

The users want to locate people by last name. They want flexible matching. That's not very hard.

The DBA wants to do some wild-card searches efficiently. The DBA may not be responding to the users actual request, making this more complex than it needs to be.

I'm not in contact with the users, so I don't know the real requirements. I'm hearing this through the DBA-filter ("all singing, all dancing, all SQL".) I may also be hearing this through IT management filter ("only use technology I recognize from my programming days".)

In my experience, wild-card searches are rarely the user's first choice. They want more flexible matching. While the SQL LIKE-clause is one solution that might work, it is rarely what the users really want.

The DBA knows that the SQL LIKE-clause effectively defeats indexing and forces row-by-row comparison. And we all know that row-by-row processing is evil.

Premature Optimization

Question 1. Is this premature optimization?

There's no way to tell. The database server may be beefy enough and the query rare enough that a basic LIKE-clause regular expression will work just fine.

Step 1. Benchmark this baseline solution.

As Fast as Possible -- in SQL

One way to find names quickly is to denormalize the data base. In addition to the proper names, also store the soundex of the name. Since this is stored, and there's no function call in the WHERE clause, and this is fully indexed, it will find "similar-sounding" names very quickly.

Soundex has limitations, so some folks use metaphone. The principle is the same. When inserting or updating the name, also insert (or update) the metaphone of the name.

This, BTW, does not involve any wild-card. Except in unusual cases, it always returns a set of candidates. And the set of candidates is a better fit than any wild-card search. More focused, and the whole name is considered.

Step 2. Prototype the soundex solution. It's hard to explain, and impossible to visualize. Actual result sets make it concrete.

Throw Memory At It

Here's an alternative that works really well.

Stop using the database.

Don't waste brain cells trying to write this kind of super-flexible search in SQL. It's better done in code. Write a simple materialized view with name and PK and nothing else. Create the smallest possible table that can be used just for name matching -- nothing else in this table. It's little more than an index.

Write a simple web service that queries this physically small table, doing a search algorithm. The web service will locate near-matches in this small table. It could return full rows for the top matches, or simply return the names and PK's for users to pick from.

You have several candidate algorithms for this server. A wisely-written web service can use a combination of algorithms and return a match score along with the names and PK's.

An alternative web service can query the name/PK table using a nice regular expression library. Since RE syntax can be complex, you would translate from a user-friendly syntax to a proper RE syntax.

For instance, the LIKE-like syntax can be reformulated to proper RE syntax. The %'s become .* and the _'s become .'s. Or perhaps you offer your users shell-like syntax. In this case, the *'s become .* and the ?'s become .'s.

Either way, the user's wild-card becomes a proper regular expression. The web service queries the table, matching all input against the RE. The service could return full rows for the top matches, or simply return the names and PK's for users to pick from.

This little web service can be granted a large amount of memory to cache large row sets. Boy will it be fast.

Also, depending on the pace of change in the underlying table, it may be possible for this service to query all names into a cache once every few minutes. Perhaps it can do this by first making a SQL request to refresh the materialized view and then a query to fetch the updated view into memory.

What the DBA wants

The DBA wants some magical pixie dust that somehow makes a query with a LIKE clause use an index and behave like other properly indexed columns.

The actual email enumerated four of the possible ways a LIKE clause could be used. I'm guessing the hope was that somehow the enumeration of a subset of candidate LIKE clauses would help locate the pixie dust.

Here's my advice. If this magical LIKE clause feature already existed, it would be in the DBA guide. Since it isn't in the DBA guide, perhaps it doesn't exist. Enumerating four use cases (name, *name, name* and *name*) doesn't help, it's still not going to work out well. Remember, SQL's been around in this form for decades; the LIKE clause continues to be a challenge.

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About Me

Steven F. Lott is a consultant, teacher, author and software developer with over 35 years of experience building software of every kind, from specialized control systems for military hardware to large data warehouses to web service API's.